Road crews were fighting to keep up with moderate to heavy snow, sleet and freezing rain falling across central Indiana on Tuesday.

Travel restrictions were put in effect in at least 23 Indiana counties as of 6 p.m. Those counties were:

Blackford, Boone, Carroll, Cass, Clinton, Delaware, Fountain, Grant, Hancock, Hendricks, Henry, Howard, Jay, Madison, Miami, Montgomery, Randolph, Tippecanoe, Tipton, northern Vermillion, Warren, Wayne, Wells.

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Slideshow: Winter Storm Belts Indiana

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Restrictions are different, depending upon the county. In some of the counties, all but emergency travel is restricted. In some counties, drivers could be fined or towed for violating the restrictions.

Plow trucks are out in full force, but all roads, except in far southern portions of the area, were slick and hazardous. Most schools and several businesses were closed, which help to cut down on the volume of traffic on the roads in central Indiana and kept the number of crashes down.

The Indiana Department of Transportation and the Indianapolis Department of Public Works urged motorists to stay off the roads unless it was absolutely necessary to travel.

A snow emergency was declared in several counties north of Indianapolis. In those counties, only emergency vehicles were authorized to be on the roads.

"People are sliding off everywhere," said Fountain County Emergency Management Director Joe Whittaker.

In Randolph County, snowplow drivers were told to get off the roads, which had become so dangerous that the county issued a Level 3 snow emergency.

In the Indianapolis area, DPW had all of its 82 trucks on the road and placed scrapers on trash trucks. Additionally, they called on some private contractors to keep the roads cleared.

Indianapolis Mayor Bart Peterson said he was pleased with the efforts of road crews on Tuesday.

Peterson, who canceled all of his scheduled meetings and appearances to focus on the storm, visited salt crews at the DPW garage.

"They are out there in risky and dangerous situations trying to keep the streets clear for you. So, if you see them -- a salt truck or trash truck with a snowplow on it, or really any truck with a plow on it -- just treat them as an emergency vehicle and let them go past. Then, you can tuck in behind them and you will have a clearer road," Peterson said.

Peterson said the storm is the biggest the city has seen in years.

INDOT said its full complement of 335 trucks are out in central Indiana. Priority is given to interstates first.

As of noon, more than 600 closings and delays were reported to 6News, including Indianapolis Public Schools, which closed for all students and staff.

Though there were relatively few crashes, some problems were reported. A couple of issues cropped up on Interstate 65 just south of downtown Indianapolis before 6 a.m., where a jackknifed tractor-trailer blocked two northbound lanes at Raymond Street. On I-65 at Market Street, another truck was stalled in the left lane. Both of those traffic incidents were cleared a short time later.

DPW said there are several ways motorists can help keep the roads as safe as possible during the storm:

Stay at work and leave at the time you would normally leave. This helps prevent traffic jams and allows road crews to clear major roads.

Plan to use alternative routes if your normal route becomes impassable.

Drive with caution and be on the lookout for slick spots, especially on bridges and overpasses.

Snow plows should be treated as emergency vehicles, just like a fire truck or ambulance.

At Indianapolis International Airport, about a third of 45 departing flights were canceled between 6 a.m. and 8:45 a.m. Air travelers were urged to check with their airline to find out the status of their flight.